Engine systems may be configured with boosting devices, such as turbochargers or superchargers, for providing a boosted aircharge and improving peak power outputs. The use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine, but with additional fuel economy benefits. However, compressors are prone to surge. For example, when an operator tips-out of an accelerator pedal, an engine intake throttle closes, leading to reduced forward flow through the compressor, degrading compressor performance, and potentially causing compressor surge. Compressor surge can lead to NVH issues such as undesirable noise from the engine intake system.
One way of addressing compressor surge involves recirculating compressed air across an intake compressor to enable rapid decaying of boost pressure. One example of such an approach is shown by Blaiklock et al. in US 2012/0014812. Therein, a compressor bypass valve (also known as a compressor recirculation valve or CRV) is situated in a passage coupling the compressor outlet to the compressor inlet. The valve is maintained closed during steady-state boosted engine operation and actuated open in response to an indication of surge. By opening the valve, a portion of air discharged from the compressor is recirculated to the compressor inlet, thereby reducing the pressure differential across the compressor and improving compressor flow.
However, the inventors herein have identified potential issues with such an approach. While the recirculation of boosted air across the compressor improves surge margin, more turbine power may be required to drive the recirculation and move the compressor operation out of the surge region than turbine power required to continue operating the compressor without recirculation. Additional turbine power may also be required to maintain the boost pressure. For example, during soft surge conditions, surge may need to be addressed without dropping boost pressure. As such, there may be conditions when the turbine power available is insufficient to drive enough compressor recirculation to move the compressor operating point away from the surge region. Also, the turbine power may not be sufficient to enable the desired boost pressure to be maintained. While the available turbine power can be increased by increasing an exhaust pressure upstream of the turbine (such as by specifying a smaller or more restrictive turbine as part of the turbine matching exercise), increasing the turbine inlet pressure increases the engine pumping work and thereby degrades vehicle fuel economy. In addition, restricting the turbine to achieve better shaft power at low engine power conditions can lead to excessive restriction at high engine power conditions, which may limit the maximum power output of the engine.
The inventors herein have realized that a scroll valve coupled to a binary flow turbine (that is, a turbine having two distinct scrolls or volutes on the turbine inlet) can be advantageously used to change the restriction of the turbine during surge conditions and provide the increased turbine power that is required to drive compressor recirculation flow. The valve may be adjusted to allow exhaust gas to flow to one or both of the scrolls, thereby changing the turbine restriction. For example, when exhaust flows to one scroll, the turbine acts like a smaller turbine producing higher inlet pressure and more shaft power for a given shaft speed. This extra shaft power can be advantageously used to provide the compressor power necessary to flow the additional recirculated air. In comparison, when the exhaust flows to both scrolls, the turbine acts like a larger turbine reducing exhaust pumping work when extra recirculation power is unnecessary, and increasing engine peak power output. In addition, by coordinating the adjustments to the scroll valve position with adjustments to a compressor bypass valve, and/or a wastegate actuator, a surge margin can be improved.
In one example embodiment, compressor surge may be relieved by a method for a boosted engine, comprising: in response to an indication of surge, adjusting each of a first valve located in a passage coupling a compressor outlet to a compressor inlet, and a second valve coupled to an outer scroll (herein also referred to as a secondary scroll) of a multi-scroll exhaust turbine. As such, the turbine may also include a primary scroll inner to the secondary scroll. In this way, a scroll valve may used to provide the turbine power needed to drive increased compressor recirculation via a compressor recirculation valve, the combined approach enabling compressor operation to be shifted out of a surge region.
As an example, a turbocharged engine system may include a compressor driven by a multi-scroll exhaust turbine. A compressor recirculation path may be provided for recirculating a portion of boosted aircharge from downstream of the compressor to the compressor inlet. This may include warm, un-cooled boosted air from downstream of the compressor and upstream of a charge air cooler, or cooled boosted air from downstream of the compressor and downstream of the charge air cooler. An opening of a first compressor recirculation valve (CRV) located in the recirculation path may be increased in response to an indication of surge to increase an amount of compressed air that is recirculated to the compressor inlet. By enabling more recirculation, a boost pressure downstream of the compressor is decreased. The exhaust turbine may include a first, outer scroll and a second, inner scroll. A second scroll valve may be coupled to the outer scroll, but not the inner scroll. An opening of the scroll valve may be adjusted to vary an amount of exhaust gas directed through each of the scrolls. During the surge condition, a turbine power required to drive the recirculation via the first valve while maintaining boost pressure may be determined. An opening of the scroll valve may then be decreased so as to limit exhaust flow through the outer scroll. In doing so, a turbine inlet pressure, and thereby a turbine peak power is increased. The scroll valve adjustment may be adjusted to provide a turbine power that is sufficiently high to drive the recirculation of compressed air via the first valve. A wastegate coupled across the turbine may also be concurrently adjusted to balance shaft power. In particular, the wastegate opening may be decreased to reduce a portion of exhaust gas that is diverted from the turbine inlet to the turbine outlet, thereby increasing a turbine inlet pressure and controlling an engine air flow.
In this way, coordinated adjustments to the position of a scroll valve, a compressor recirculation valve, and a wastegate can be used to address surge while maintaining boost pressure and engine air demand. By using compressor recirculation valve adjustments to improve compressor flow, while using scroll valve adjustments to provide the required turbine power, a margin to surge is improved. By using concurrent wastegate adjustments to maintain turbine inlet conditions within desired limits, better shaft power can be provided to the compressor at low engine power conditions as well as at high engine power conditions, improving engine power output. By coordinating the action of the scroll valve and the compressor recirculation valve, a bigger compressor and a bigger turbine can be used to provide higher boost pressures without incurring frequent surge issues at low engine speeds. Overall, the coordinated approach enables surge margin and boosted engine performance to be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.